Today: Facebook discusses changes to its News Feed algorithm and Twitter offers a more secure authentication system as social media continues to grow. Also: Stocks fall as Apple’s (AAPL) growth stalls.

The Lead: Facebook News Feed, Twitter security get updates as social media use grows

Facebook and Twitter announced changes to their signature offerings meant to increase interaction and offer more security, respectively, as the use of social media continues to increase among adults in the United States.

Facebook announced Tuesday that it will be more open about changes to its News Feed, the Menlo Park company’s core offering to users on both mobile and desktop platforms. The first change that is part of Facebook’s avowed openness will pull older posts closer to the top of a user’s feed if the user has not seen those posts before, an effort to avoid repetition but also help Facebook’s business partners ensure their posts are seen.

“There’s a fundamental tension here because, on the consumer side, people are going to use the News Feed for some number of minutes a day, but not all items can possibly or conceivably be consumed. On the other side, you have publishers who ask ‘Why aren’t people seeing my stuff all the time?’,” Facebook executive Chris Cox said at Tuesday’s event.

A second change, that is not yet implemented, will place more weight on recent interactions when deciding what posts a user sees, an attempt by Facebook “to capture your current state of mind,” according to News Feed ranking engineering manager Lars Backstrom.

The move comes a day after a Pew study showed that 72 percent of online U.S. adults are active on social-networking sites, up from 67 percent late last year and a gigantic jump from 8 percent in 2005, when Pew first began researching social-media usage. Seniors’ usage of social media was a large part of that growth, with 43 percent of online U.S. adults ages 65 and older saying they use social networks, up from just 13 percent in a 2010 study.

Pew’s report focused on San Francisco-based Twitter, finding that the microblogging service is now used by nearly one in five U.S. adults, 18 percent. That percentage has more than doubled from 8 percent since Pew began studying Twitter usage in November 2010.

In an attempt to maintain its growth in the face of recent high-profile hacking attacks, Twitter announced a novel new authentication program Tuesday for its mobile service. After several users, including The Associated Press, had their accounts hacked, the company announced it would begin requiring a two-step authentication process, which requires users to respond to a text message to ensure they are the account’s owner. In its new system, however, Twitter is storing all authentication on its own system and using a complicated system to ensure the proper user is accessing the account, cutting down on steps required by the user.

“Other two-factor systems rely on a shared secret. We wanted to come up with a design where it is only stored on the client side; the secret’s only stored on the phone,” Twitter security engineer Alex Smolen explained to Wired reporter Mat Honan, who was famously victim of a hacking attack last year.

With their accounts secure, Twitter users can go back to tweeting about television: TV-ratings giant Nielsen announced Tuesday that Twitter users can boost ratings of shows based on the number of tweets sent about a live program. The first verification of that kind could help the San Francisco company attract more advertising dollars as it heads toward an initial public offering.

Facebook has only recently managed to top its IPO stock price after more than a year on the market, and prices stayed above that $38 level Tuesday despite a 1.6 percent decline, with Facebook closing at $38.55.

Apple, which has experienced a resurgence on Wall Street and regained the title of most valuable company in the world, dropped 0.9 percent to $465.25 Tuesday after announcing a deal for consumers. Apple said Tuesday it would sell chargers for its mobile devices for only $10 when customers bring in a third-party charger, an effort on the Cupertino firm’s part to cut down on possibly dangerous chargers such as the one blamed in the death of a Chinese woman last month. After Apple’s important victory over Samsung with a White House veto last weekend, the two companies are preparing to return to court in their San Jose legal skirmish Friday, with Samsung’s products in danger of being banned this time.

Intel (INTC) dropped 0.5 percent to $22.80 after IBM enlisted Silicon Valley heavyweights Google (GOOG) and Nvidia in an open-source project that could hurt the Santa Clara chipmaker. IBM didn’t profit from the move, however, as Big Blue dropped 2.3 percent to $190.99 after a rating downgrade; Google declined 0.9 percent to $896.57 while fighting its own patent battle with Microsoft, and Nvidia fell 1.5 percent to $14.61. Netflix (NFLX) was one of Silicon Valley’s few gainers Tuesday, as the Los Gatos video-on-demand company moved 0.8 percent higher to $255.90 after securing rights to a new original offering based on Marco Polo’s adventures.

After Tuesday’s trading session, Solta Medical announced that CEO Stephen Fanning had resigned after a poor quarter that forced the Hayward company to pull down its forecasts, with Chairman Mark Sieczkarek taking over as interim CEO. Solta is the third SV150 company to lose its CEO in the current earnings season, after Polycom CEO Andrew Miller left amid controversy and Juniper CEO Kevin Johnson said he would retire when a replacement is found. Solta shares declined 4.9 percent to $2.51 in Tuesday’s regular session.

In other late announcements from SV150 companies, Sunnyvale networking firm Finisar announced preliminary quarterly revenues that surpassed the top of its forecast range, sending the stock up nearly 20 percent in after-hours trading. San Jose’s Super Micro Computer increased profits 52.9 percent year-over-year to 19 cents a share and boosted revenues 16.8 percent to $322.3 million, while chip manufacturer Monolithic Power Systems earned 14 cents a share on revenues of $51.5 million. Pericom Semiconductor lost 74 cents a share on revenues of $31.7 million, while Hercules biotech company Bio-Rad increased revenues but saw its profits drop.

And the widely watched Standard & Poor’s 500 index: Down 9.77, or 0.57 percent, to 1,697.37

Check in weekday afternoons for the 60-Second Business Break, a summary of news from Mercury News staff writers, The Associated Press, Bloomberg News and other wire services. Contact Jeremy C. Owens at 408-920-5876; follow him at Twitter.com/mercbizbreak.